A light-emitting diode (referred to hereinafter as LED) represents one of the most popular light-emitting devices today. In recent years, the luminous efficacy of LEDs, defined as lumens per Watt, has increased significantly from 20 lumens per Watt (approximately the luminous efficacy of an incandescent light bulb) to over 200 lumens per Watt, which greatly exceeds the luminous efficacy of a fluorescent light at 60 lumens per Watt. In other words, for a fixed amount of light output, LEDs consume only one third the power compared to fluorescent lights, and one tenth compared to incandescent light bulbs. Accordingly, it is not surprising today that lighting fixtures with LEDs is have recently been replacing incandescent light bulbs and fluorescent light tubes. A new term “Solid-State Lighting” has been created. The term “Solid-State Lighting” refers to the type of lighting that uses semiconductor light-emitting diodes, such as an LED rather than traditional light sources.
In the field of solid-state lighting, most of the light sources are white light. The white light sources may not be evaluated only in terms of luminous efficacy. Correlated color temperature (referred to hereinafter as CCT) and color rendering index (referred to hereinafter as CRI) may be other parameters that are widely used to compare and evaluate light sources. The CCT of a fight source indicates the relative color appearance of the particular light source on a scale from “warmer” (more yellow/amber) to “cooler” (more blue) light. CCT units are generally given in Kelvin or K. CCT is the color temperature of a theoretical black body radiator that to human color perception most closely matches the light from the lamp. For example, the CCT of a lit match may be approximately 1700K and the CCT of daylight may be approximately 6500K, whereas the CCT of a CRT monitor may be approximately 9000K.
On the other hand, the CRI is a quantitative measure of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source. The CRI is calculated by comparing the color rendering of the test source to that of a theoretical “perfect” source, or a black body. CRI is a unit-less quantity. Higher CRI values correspond to better color rendering with a maximum value of 100, while lower values correspond to poorer color rendering with a minimum value of zero. Light sources with higher CRI values are desirable in color-critical applications, such as to photography. Depending on the application, the CRI goal is typically in the range of 75 to 95. For example, the CRI of an incandescent light bulb may be 100, but the CRI of a fluorescent tube may be 75. The CRI of most white LEDs may be less than 90.
Two light sources that have identical CCT values may render a given object's colors very differently, due to differences in the spectrum of the light, which is also referred to as spectral content. The CRI value of a light source is intended to capture the significance of such differences by denoting how accurately a particular light source renders the colors of illuminated objects. For example, a first light source comprising tri-colored RGB LEDs (red, green and blue light emitting diodes) and a second light source comprising a similar tri-colored RGB LEDs and an additional amber LED may be configured to produce a visible light having the same or substantially the same CCT value. However, the CRI of the first and second light sources may be different, because the spectral content of the two light sources are different.
One observation is that the light source having a more distributed spectrum may produce visible light with a higher CRI value. The spectrum of the first light source discussed above may include three narrow bands of waveforms peaking at or near the red, green and blue light regions, respectively. In contrast, the spectrum of the second light source discussed above may include four narrow bands of waveforms peaking at or near the red, amber, green and blue light regions, respectively. Accordingly, the second light source may be considered as being more distributed or “richer” compared to the first light source, and thus, having a higher CRI value. Similarly, a third light source comprising a blue light-emitting die that is covered with red phosphor may produce a similar visible light, but have a higher CRI value when compared to the first light source. This phenomenon may occur because the third light source may have a “richer” or more distributed spectrum. However, it will be appreciated that the computation of a CRI value involves several precise and complex mathematical steps. Thus, the CRI of any particular light source one cannot be arbitrarily predicted.
The spectral content of a light source is determined by the particular selection of light-emitting dies and phosphors or wavelength converting encapsulant materials utilized. There has been extensive research performed on the precise selection of light-emitting dies and the creation of phosphor recipes to produce white light sources with high CRI values. For example, a white light source produced utilizing a light-emitting die emitting blue light or ultra-violet (referred to hereinafter as UV) encapsulated with a mixture of yellow and red phosphor will produce white light with a high CRI value. The light-emitting die emitting blue light or ultra-violet light may be referred to as a near-UV light source. However, the CRI of such visible light sources may not be tunable. Once the light source leaves the manufacturing facility, the CRI of the light source is typically fixed.
In some applications, such as when a light source is being used for mood lighting or decorative lighting, a light source with a tunable CRI may be desirable, in order to change the atmosphere or mood in the environment for different times of day or occasions. For example, light sources with controllable CRI may be desirable in restaurants, bars, hotels, and shops, among other situations, in order to modify the lighting conditions of the tables, clothes racks, display cases, etc., according to the atmosphere, mood, time of day, season, or object to be illuminated. Similarly, a light source with a tunable CRI may be desirable for other applications, such as lighting for aquariums so that the lighting may produce different effects at different times of a day.